Ph.D. Defense: James Sanders
CBEE Ph.D. Defense Announcement
Candidate: James Sanders
March 15, 2018; 2 pm; TRC 206
Dissertation Title: A Novel Equilibrium Passive Sampler for Methylmercury and Other Advances in Benthic Bioavailability Monitoring and Reduction
Abstract: The ecological and human health risks posed by persistent sediment contaminants like polychlorinated biphenyls (PCBs) and methylmercury (MeHg) are often controlled by their bioavailability at low trophic levels. Benthic and epibenthic organisms can serve as a conduit for the contaminants to enter aquatic food webs, where they are subject to a large degree of biomagnification. As a result, benthic bioavailability is of central importance. Technological developments in recent decades have improved our capabilities for both monitoring and reducing benthic bioavailability. This dissertation details a group of projects linked by the common goal of advancing bioavailability science to improve the quality of contaminated site risk assessment and management.
A multi-year, pilot-scale study was performed to evaluate the persistence and efficacy of several different activated carbon (AC)-based sediment amendments in the unique setting of an intertidal, estuarine marsh overrun with Phragmites australis reeds. Black carbon and PCB measurements showed that all three tested amendments remained in place through three years of daily tidal cycling and an unusually disruptive storm event. All three reduced bioaccumulation of PCBs by benthic and epibenthic organisms, with the most effective being a product containing a finer-grained AC. The passive sampling effort for this project employed several different polymer types and performance reference compound (PRC) adjustment methods, enabling direct comparisons that can inform ongoing method development and standardization. The results indicated good agreement among disparate PRC methods, but also identified AC amendment as a potential confounding factor for a newer method based on modeling diffusive transport through sediments.
For mercury and MeHg remediation, a project was undertaken to explain the large variability observed in early tests of AC amendment for mercury-impacted sediments. AC-water partitioning constants for mercury species complexed with dissolved organic matter (DOM) were measured and found to be far smaller than those previously reported for chloride complexes. Accordingly, the positive effect of AC on inorganic mercury partitioning in microcosms of slurried soil was attenuated by the addition of exogenous DOM. However, no effect on MeHg partitioning was observed. These results highlight the importance of considering site chemistry in remediation design.
Lastly, a novel passive sampling device for MeHg in sediment and soil porewaters was developed. This project seeks to address the need for a reliable, equilibrium sampling strategy capable of predicting MeHg bioavailability. Prospective materials were tested under increasingly environmentally realistic conditions and several proved capable of concentrating aqueous MeHg by three to four orders of magnitude, permitting good prediction of porewater concentrations in a variety of matrices including salt marsh soils with and without AC amendment. One sampling material, a suspension of AC particles in agarose gels, underwent additional testing that found reasonably rapid, internally diffusive, and reversible MeHg accumulation. Sampler-water partitioning was in the range of 10^3.0–10^4.0 L kg-1, comparable to sediment-water partitioning at many sites. In a preliminary bioaccumulation study, sampler uptake was correlated with concentrations in the benthic amphipod L. plumulosus, which were reduced by amendment with AC. The project constitutes a successful proof of concept and provides the basis for further refinement. Future work will seek to optimize the design and preparation of samplers and develop a more detailed understanding of the dynamics of MeHg exchange among samplers and various ligands to facilitate better interpretation of their measurements.